EE Version 2.11.7 - 500 Internal Server Error after Publishing Entry

We recently upgraded our EE2 clients to the latest version 2.11.7 After logging in and publishing/editing an entry the site goes down with a 500 Internal Server Error (front end and control panel.) After clearing cookies the site and CP come back up, and the change was successfully made.

This is happening to ALL the clients we upgraded (several of them), so we can confirm it is not limited to a specific extension or module error.

If anyone can help it’s much appreciated! Thanks